1. Field of the Invention
The present invention is in the field of pressure transducers having a variable capacitance between a diaphragm exposed to the pressure and a fixed electrode.
2. Description of the Related Art
Capacitance diaphragm gauges (CDGs) have been used for many years to measure pressures. CDGs are particularly useful for measuring very low pressures (e.g., much lower than atmospheric pressure) such as pressures in an evacuated system (e.g., a semiconductor fabrication system). A CDG produces an electrical output that represents a measure of a pressure input with respect to a reference pressure.
Basically, an exemplary CDG includes at least one electrode that is supported on a suitable support structure. The electrode is positioned in close proximity to a flexible diaphragm in a sealed and evacuated cavity. The diaphragm is positioned in the device so that one face of the diaphragm (the pressure face) is exposed to an unknown pressure to be measured. The electrode is proximate to the opposite face of the diaphragm (the electrode face). The unknown pressure on the pressure face is measured relative to a reference pressure on the electrode face. The reference pressure is substantially constant within the sealed and evacuated cavity. The diaphragm and the electrode comprise the two plates of a variable capacitor that has a capacitance the varies in response to deflections of the diaphragm caused by pressure variations.
In many applications, the CDG is positioned within a suitable housing of a pressure-measuring device with the pressure face of the diaphragm exposed to the unknown pressure via suitable passages. Alternatively, the pressure face of the diaphragm may be exposed directly to the unknown pressure. For example, the CDG may be mounted such that the pressure face of the diaphragm is in a gas flow conduit, in which case it is preferable that the diaphragm and other portions of the CDG do not extend into the gas flow to partially block the gas flow or to cause turbulence in the gas flow. If no portion of the CDG extends beyond the pressure face of the diaphragm, the pressure face can be mounted substantially flush with an inner wall of the gas flow conduit. A CDG having such a configuration is called a flush diaphragm design. One skilled in the art will appreciate that a flush diaphragm CDG can be welded into a housing to make a more general device. On the other hand, a CDG that does not have flush diaphragm generally is not convertible to be used in applications requiring a flush diaphragm device because the outer support structures for the diaphragm extend beyond the pressure face of the diaphragm.
CDGs having flush diaphragms are known in the art. For example, a first type of flush diaphragm CDG is machined out of a solid block of suitable material to leave a thin layer of material at one end of the block to form the diaphragm. In some cases, the material may be heat treated for certain desired results or because of the properties of the material.
Another known type of flush diaphragm CDG is called corrugated diaphragm CDG. The corrugated diaphragm has waves formed into the surface to cause extra material to be present in order to produce more linear deflections in response to the applied pressure. The diaphragm for this type is usually welded into place.
A third type of flush diaphragm CDG has a diaphragm formed from a thin material. The thin material is highly tensioned in some manner and is welded in place.
Much emphasis is placed on the hysteresis characteristics of a finished pressure measuring device. Hysteresis refers to the differences between the output of the transducer on approaching a given pressure from different directions (i.e., approaching the given pressure from higher pressures as the unknown pressure is decreasing versus approaching the given pressure from lower pressures as the unknown pressure is increasing). Although the same output value should be generated for the given pressure irrespective of the previous pressure, hysteresis effects may cause the output value to be too high when the given pressure is approached from a higher pressure and may cause the output value to be to low when the given pressure is approached from a lower pressure.
The maximum value of the hysteresis error is usually at the midpoint of the pressure excursions. An excursion from zero pressure to full-scale pressure is the maximum normal excursion. Abnormal excursions can cause greater errors. Since hysteresis errors depend at least in part on the magnitude of the pressure excursions, the hysteresis errors are usually unpredictable and are therefore major concerns. In contrast, other errors, such as, for example, linearity or temperature errors, are more correctable because they are repeatable and therefore predictable.
A diaphragm subjected to pressure has to carry the pressure load. The difference between the pressures applied on the opposite faces of the diaphragm causes a deflection of the diaphragm. The electrode face of diaphragm acts as one plate of a variable capacitor having the electrode as the other plate of the capacitor. If additional electrodes are included, multiple capacitors are formed with the electrode face of the diaphragm forming one plate of each capacitor. The deflection of diaphragm moves the diaphragm closer to or farther from the electrode, thus varying the capacitance. The capacitance is determined in a suitable conventional manner to provide a measurable quantity responsive to the pressure applied to the pressure face of the diaphragm.
In order to produce repeatable measurements of the unknown pressure, the diaphragm deflection should occur with a minimum of hysteresis. That is, when the pressure returns to the previous magnitude, the diaphragm should return to its previous state of deflection regardless of whether the pressure initially increased and then decreased or initially decreased and then increased.
Reduction of hysteresis has been accomplished by carrying the load in tension. It has been found that smaller changes in the magnitude of the tension in response to pressure changes results in less hysteresis and thus results in greater measurement accuracy. One problem with high pressure measuring devices is to keep the deflection small enough by having a pretension carrying the load.
Many techniques have been used to pretension diaphragms, particularly for diaphragms in low pressure CDGS; however, the techniques used for high pressure diaphragms have proven to be very limited, and as the devices have become smaller, the techniques have become even more limited. One technique that has been used to pretension a diaphragm is to heat the diaphragm prior to welding the diaphragm to the body of the CDG so that when the diaphragm cools, the diaphragm will shrink and develop tension. Previous attempts to do pretension a diaphragm with this technique consisted of placing the diaphragm in contact with a heated platen. This technique causes the whole fixture to become hot and thus causes a significant uncertainty in results as sequential units are processed. Such a technique also presents problems in maintaining good thermal contact between the diaphragm and the platen, which again causes the resulting tension on the diaphragm to be nonrepeatable.
The support structure in a typical CDG is formed as one piece with a portion of the structure proximate to the diaphragm providing the function of a shim that spaces the diaphragm away from the electrode in its rest or zero position. Forming the shim as part of the CDG body is a very expensive and unrepeatable way to obtain the spacing between the diaphragm and the electrode. For example, the thin lip of the shim needs to be machined in with great care to provide the tolerances that are necessary to produce a repeatable initial zero capacitance. The shim is under great pressure when the diaphragm deflects. Therefore, the shim needs to be extremely hard. In order to obtain the required hardness with the one-piece design, the part is heat-treated after machining. The heat-treating may cause the part to warp and to lose the spacing accuracy that is required for precision measurements.